Vertical surface-emitting lasers (VCSEL) are known in the art for low manufacturing-cost. Recently, vertical surface-emitting lasers with an external cavity (VECSEL) were developed, which allow large-area beams to be generated efficiently with high beam quality. By placing a frequency conversion device in the external cavity it is now possible to generate laser light at wavelengths, for which a direct generation of laser light is not possible at reasonable cost and effort.
A typical field of application of such a device are RGB image projectors, for which laser light sources are ideal due to their high luminance and high beam quality. While red and blue laser light can be generated directly by laser diodes at a reasonable cost, it is currently not feasible to generate green laser light directly using a laser diode. Therefore, vertical surface-emitting external cavity laser devices with intracavity frequency conversion are promising for this application.
Within vertical surface-emitting external cavity laser devices with intracavity frequency conversion, laser light is generated in a semiconductor laser element, similar to the laser chip of a VCSEL device, comprising at least one gain layer with usually several quantum wells and at least one highly reflective distributed Bragg reflector. A laser mirror is arranged spaced from the laser element to form an external cavity, which completes the laser resonator. When the laser element is operated, this setup generates light at a fundamental frequency. By placing a frequency conversion device, for example a non-linear crystal, in the external cavity, light at a second frequency being different from the fundamental frequency, e.g. at a harmonic frequency, is generated. The light at the second frequency can then easily be coupled out of the external cavity and be used for the desired application.
A problem arises, since the performance of both the semiconductor laser element and frequency conversion device typically are temperature dependent. The conversion efficiency of the frequency conversion device from the fundamental frequency to the second frequency strongly depends on an exact match of its phase-matching bandwidth, which is strongly temperature-dependent for most of the common efficient frequency conversion materials. If the gain bandwidth of the laser element drifts due to a change of the temperature, the gain bandwidth may drift out of the phase-matching bandwidth, which may result in low conversion efficiency and thus low power of the light at the second frequency. Usually, this problem can be addressed by an active temperature control of both, the laser element and the frequency conversion device.
Document WO 2006/105249 discloses a frequency stabilized vertical extended cavity surface-emitting laser, which comprises a semiconductor laser chip for generation of laser light at a fundamental frequency, an output coupler, defining an external cavity and a non-linear crystal for generating light at the second-harmonic frequency arranged within the external cavity together with a thin-film interference filter.
Here, the thin-film filter serves to constrain the emission wavelength of the laser at the phase-matching bandwidth of the non-linear crystal. Although due to this setup, a temperature control of the laser chip can be omitted, it is still necessary to actively control the temperature of the non-linear crystal, which is costly and may lead to significant problems when integrating the laser device in small projection units.
It is therefore an object to provide a surface-emitting external cavity laser device with intracavity frequency conversion, which is universally applicable and may be manufactured cost-efficiently.